The objective of this proposal is to identify specific mechanisms governing the role of inhibition in temporal coding. A temporal code uses precisely timed events to represent information. For example, sound localization in birds, mammals, and other vertebrates relies on temporal codes in which the events are spike times. In this case, neurons first fire spikes at specific times relative to the time at which the sound arrives at each ear. Then, time-comparing neurons integrate those spike times such that specific cells only respond when sound comes from certain locations. Temporal coding is also important for recognizing features of human speech such as vowel sound duration, phoneme discrimination, and pause length;all of which involve very precise, small differences in timing. This proposal is specifically interested in the role of inhibition in temporal codes used to process very small differences in spike times. Although it is clear that temporal codes are important for sensory processing, we know relatively little about the cellular mechanisms underlying temporal codes. The relevance of this deficit can be seen in our inability to separate out the many causes of Central Auditory Processing Disorder (CAPD), a group of ill-defined auditory deficiencies resulting from central nervous system dysfunction. The experiments proposed here will take an important step towards elucidating human auditory processing by specifically addressing the role of inhibition in temporal coding. The proposed research will test the hypothesis that inhibition not only generates sensitivity to spike timing differences, but also shapes the representation of that sensitivity by altering how neurons are tuned. Specifically, this research will analyze the role of inhibition at three distinct locations in a neural circuit: the cells providing input to the time-comparator neurons, the time-comparator neurons themselves, and the cells that integrate the outputs of time-comparator neurons. First, a combination of immunohistochemistry and electrophysiology will be used to assess if inhibitory inputs onto inhibitory cells affect sensitivity to spike timing differences. Then, a novel in vivo extracellular axon recording approach will be used along with a pharmacological blocker of inhibition to provide a direct test of how the presence or absence of inhibition affects the tuning properties of time-comparator neurons. Finally, immunohistochemistry and electrophysiology will be used to assess the timing, strength, and location of excitatory and inhibitory inputs onto the neurons that integrate the outputs of time-comparator neurons. All of these experiments will be done using weakly electric fish as a model system because their sensory processing uses small spike timing differences to encode behaviorally relevant signals in an accessible neural circuit devoted exclusively to this sensory modality. The experiments in this proposal will provide critical insight into the diverse roles of inhibition in temporal coding, thus leading to an improved understanding of neural processing of auditory information. PUBLIC HEALTH RELEVANCE: The research described in this proposal is relevant to how the brain processes sensory information, particularly sound duration and location. Specifically, it will analyze how inhibitory and excitatory connections converge onto brain cells that use the timing of those signals to mediate perception of stimuli with very small timing differences. The results will advance our understanding of time-based neural coding.